1. Field of the Invention
The invention relates generally to measuring devices and to devices for measuring the pressure in a sealed container. More particularly, the invention relates to a non-invasive device and method for measuring the pressure of a gas in a double-envelope lamp.
2. Prior Art
An infrared gaseous discharge lamp of integrated double-envelope construction has an inner chamber or envelope filled with a gaseous medium under relatively high pressure which provides illumination when the lamp is energized. The outer chamber or envelope is normally evacuated or otherwise provided with a relatively low-pressure gas.
Double-envelope lamps are subject to gas leaks from the inner chamber to the outer chamber. Eventually, these leaks may lead to catastrophic lamp failure by a mechanism that involves electric arcing in the outer chamber. A device that will measure the leakage of gas to the outer chamber is necessary to determine the operational status of the lamp and to predict the storage and operating lifetimes.
Under field conditions, standard vacuum measuring devices, such as thermocouples and ionization gauges, are not suitable for lamp pressure determination because they interfere with or permanently damage the operating system. For example, integrated invasive vacuum gauges such as ionization gauges will interfere with the gas dynamics and/or complicate the engineering of the lamp's operating system. Use of a stand-alone invasive measuring device results in damage to the lamp.
The above conditions are true in glow discharge lamp systems, especially systems with a double-envelope design. A simple modification of the lamp to allow for pressure measurement is not possible. Yet, the lamp is subject to gas leaks from the inner chamber to the outer chamber, resulting in damage as noted above.
Non-invasive means are known in the prior art for measuring the pressure within a closed container. For example, a short burst of high-voltage rf power and a flash of high intensity light are employed in Pfaff et al. (U.S. Pat. No. 4,546,319) to test the vacuum in a vial, causing an ion current, the amplitude of which is indicative of pressure. Pfaff et al. indicate that in the known prior art, evacuated vials were tested by a sustained application of high voltage to ionize the gas therein, and determining the degree of vacuum by the color of the ionized gas.
In Tittmann et al. (U.S. Pat. No. 4,869,097), the pressure of the gas in a sealed vessel is measured by applying frequency-swept sonic energy to the vessel, the resonant frequency being noted to determine pressure. Eesley et al. (U.S. Pat. No. 4,452,071) uses sonic energy to measure pressure in a halogen-filled lamp, by measuring the speed of a shock wave within the lamp.
Fukushima (U.S. Pat. No. 4,402,224) tests the vacuum pressure of an evacuated electrical device which emits an electrical field, the strength of which varies with internal pressure. This field is converted to polarized light and a measurement made to determine pressure. Internal pressure in a vessel is measured in Shibasaki (U.S. Pat. No. 4,313,171) by electromagnetic inductive sensing of physical characteristics of the vessel surface.
However, the inventor is not aware of any means for the non-invasive measurement of the internal pressure of a double-envelope glow discharge lamp.
The non-invasive pressure measuring device of the present invention is based on the pressure dependence of the luminance of a gas discharge when the gas is under low pressure. For the double-envelope construction lamp, if gas from the inner chamber has leaked into the outer chamber, and this gas in the outer chamber is energized, the luminance of the gas in the outer chamber is proportional to the actual pressure in the outer chamber. For a fixed-geometry container and radio frequency (rf) voltage, the positive column discharge is uniform, and the light distribution is unchanged. The intensity of the luminous glow discharge is known to be directly proportional to the pressure of the gas at low pressures, where the discharge and gas behave ideally. Thus, measuring the luminance of a gas discharge as a function of pressure for a fixed electrode separation yields a specific calibration curve. An absolute reference is established with the measurement of a known pressure of the gas in question. Comparison of the measured signals to the calibrated luminance yields a pressure.
Measurement of the luminance of a high frequency, rf glow discharge is a new approach to pressure determination. Luminance measurements are of limited sensitivity, but the sensitivity is adequate for detecting the failure pressure of a double-envelope lamp. With only the outer chamber pressure of interest in detecting the failure of a double-envelope discharge lamp, using an rf discharge to facilitate detection in a sealed system is advantageous because it can be limited to measure only the outer envelope through the electrode placement and operation. This rf discharge can be maintained at pressures lower than those required for a dc discharge, extending the dynamic range of the measuring device.